This invention relates generally to the field of hardeners for cements and mortars and more specifically to an improved method of making a condensed aluminum phosphate hardener.
Water glass cements and mortars consist of mixtures of water glass (which in this connection maybe understood as amorphous, water soluble alkali metal silicates, mostly sodium and potassium silicates with a mole ratio of SiO.sub.2 to M.sub.2 O of greater than one and for all practical purposes, greater than two, where M represents the alkali metal) and acid resistant fillers, such as quartz sands, clays, barium sulfate, etc. The water glass is usually added as a solution; however, it can also be added as a dry powder with water being added to the dry mixture to make up the cement.
Water glass cements have been used since late in the last century. At that time, such cements consisted mainly of a mixture of sodium water glass solutions and quartz flours and sands. The hardening of these cements took weeks and months and depended to a great extent on the reaction of the carbon dioxide in the air with the water glass as follows: EQU (M.sub.2 O).sub.X.(SiO.sub.2).sub.Y +X.CO.sub.2 .fwdarw.X.M.sub.2 CO.sub.3 +Y.SiO.sub.2
where M is the alkali metal. Water glasses are polymers with the molecular weight in solution dependent upon concentration, temperature, and the ratio of M.sub.2 O to SiO.sub.2. The absolute value of X can be anywhere from 4 to more than 100. Of significance is only the ratio of Y to X which normally is .gtoreq.2.
Insoluble silica is precipitated by the reaction which holds the fillers together. These cements were highly resistant to acids, but were very porous and not resistant to water. In the mid 1920's, the first water glass cements with additions of hardeners were prepared by adding sodium or potassium fluosilicates to the fillers. Theoretically, any acid bearing material will precipitate silica from a water glass solution as follows: EQU (M.sub.2 O).sub.X.(SiO.sub.2).sub.Y +2X H.sup.+ .fwdarw.2X M.sup.+ -Y.SiO.sub.2 +X H.sub.2 O
and specifically EQU Na.sub.2 O.3SiO.sub.2 +2HCl.fwdarw.2NaCl+3SiO.sub.2 +H.sub.2 O
However, most acid materials react much too fast to be of any practical value because the cements would set the very moment the ingredients are mixed together.
The sodium and potassium fluosilicates proved themselves very practical over the years and are still used in many places. However, these compounds possess some inherent drawbacks. Besides being poisonous, they will release hydrofluoric acid fumes when used in acid service. These fumes are highly corrosive to otherwise acid resistant equipment, including stainless steel, glass and ceramics. The reaction products of sodium and potassium fluosilicates with the water glass are also 85% water soluble, which tends to increase the porosity of the cement which is another undesirable side effect.
Therefore, other materials were tried, for instance, such as disclosed in U.S. Pat. No. 2,662,022. One compound mentioned in this patent, formamide, is still used commercially. However, these materials also have some deficiencies, including poor storability.
A different type of hardener is based on condensed aluminum phosphates. U.S. Pat. Nos. 3,445,257 and 3,943,231 disclose the manufacturing and use of such hardeners. These types of hardeners, to our knowledge, are superior to any other hardener used at the present time, because they are non-poisonous, very stable in storage, and result in cements and mortars with superior properties. These hardeners are widely used.
As good as these condensed aluminum phosphate hardeners are, the known methods used to make them are tedious and require large amounts of energy. First, an aluminum orthophosphate solution is prepared by dissolving aluminum hydroxide in phosphoric acid. Then, the solution has to be heated until all the water has been removed. This step alone requires more than 50,000 BTU's for every 100 pounds of material. U.S. Pat. No. 3,445,257 advises to dry and heat the reaction mass either in bulk or in a spray dryer. Either method produces a rock hard intermediate product which has to be ground in heavy duty equipment such as a hammer mill or an edge runner mill. Although U.S. Pat. No. 3,943,231 discloses a one-step manufacturing process, this process consumes even more energy and the reaction parameters are hard to control. Because a 100-200% excess of phosphoric acid has to be used in these known processes, the mixture attacks the equipment it is contained in at the high temperatures necessary for the evaporation. Only a few rather costly materials can withstand hot concentrated phosphoric acid (e.g.: tantalum, silver, platinum).
U.S. Pat. No. 3,801,704 discloses a process for preparing condensed aluminum phosphate for catalysts, heat-resistant materials and antirusting agents using a two stage process. Ammonium phosphate and aluminum hydroxide can be used. The first step produces a wet, semisolid intermediate product which is dehydrated to form a crystalline product having an X-ray diffraction pattern with a high peak at 2.theta.=11.2.degree.. The first stage is carried out with agitation at tempratures of 90.degree. C. to 400.degree. C., and preferably 250.degree. C. to 300.degree. C. for 1 to 2 hours. The second stage is carried out between about 200.degree.-400.degree. C. for more than 3 hours.
The subject of our present invention is an improved method of preparing condensed aluminum phosphate cement hardeners which has advantages over the methods which were previously known. We have found that these materials can be prepared by an energy efficient, non-corrosive, solid state reaction of aluminum hydroxide with ammonium phosphate powders to provide an easily friable product without the need for expensive equipment. The x-ray diffraction pattern does not show a peak at 2.theta.=11.2.degree. so that the product differs from the materials described in U.S. Pat. No. 3,801,704. The X-ray diffraction pattern has a high peak at about 2.theta.=16.1.degree. and is similar to commercial condensed aluminum phosphate hardeners in this respect although the overall patterns are different.